Chapter 15 Flashcards
(10 cards)
Regulation of glycolysis: muscle
Hexokinase
Hexokinase is inhibited by its product, glucose 6-phosphate
Excess G 6-P means no need for energy production via glycolysis
Regulation of glycolysis: muscle
Phosphofructokinase (PFK)
ATP is an allosteric inhibitor of PFK
Regulatory site distinct from catalytic site
Binding lowers affinity for fructose 6-phosphate
AMP reverses inhibition, so ATP: AMP ratio is important
Regulation of glycolysis:PFK vs Hexokinase in muscle
Phosphofructokinase is more important regulatory step
G 6-P also used for synthesis of glycogen
May want to generate glycogen if no need for ATP
First committed step to pyruvate is phosphofructokinase
Regulation of glycolysis in the liver: Hexokinase and glucokinase
- HK I is expressed in all tissues, to different levels
- HK IV (glucokinase) is only expressed in the liver
- –Has higher Km, so responsive to higher [glucose]
- –Not inhibited by glucose-6-phosphate, so can function at higher [glucose]
- –Functions to clear blood glucose at higher [glucose] for storage as glycogen
Regulation of glycolysis in the liver: Phosphofructokinase
When blood glucose is high, F-6P levels increase and F-2,6-BP is made
F-2,6-BP affects activity of phosphofructokinase:
- -Increases affinity of PFK for fructose 6-phosphate
- -Decreases inhibition by ATP
Get increased glycolysis in liver when glucose is abundant
–Feedforward stimulation
Glucose transport out of a liver cell
Glucose transporter (T2) transports glucose to GLUT2 which moves it into the capillaries
Regulation by Fructose 2,6-Bisphosphate
-The levels of F26BP regulate glycolysis vs. gluconeogenesis
-F26BP activates PFK1(glycolytic enzyme)
-F26BP inhibits fructose 1,6-bisphosphatase (gluconeogenetic enzyme)
Go glycolysis if F26BP is high
Go gluconeogenesis if F26BP is low
F 2,6 BP rises and falls in parallel with blood glucose levels
Levels of F 2,6 BP are regulated by
a bifunctional enzyme (PFK-2/FBPase-2)
Regulation of the bifunctional enzyme by hormonal control (insulin vs. glucagon)
glucose levels rise, insulin is produced, PFK-2 is activated and F26BP is high which stimulated glycolysis and inhibits gluconeogenesis
glucose levels low, glucagon is produced, FBPase-2 is active, F26BP is low which inhibits glycolysis and stimulates gluconeogenesis
Glycolysis and gluconeogenesis: reciprocal regulation
Gluconeogenesis and glycolysis do not happen at the same time
-One pathway active, one inactive
Glycolysis responds to concentration of glucose
Gluconeogenesis responds to concentration of lactate and other precursors
Glycogen breakdown
Glycogen stores can be broken down to free glucose for glycolysis
The main enzyme that catalyzes the breakdown of glycogen is glycogen phosphorylase.
Other enzymes have transferase and debranching activities
Regulation of glycogen breakdown in muscle and liver cells is controlled by epinephrine and glucagon, respectively.
